As a scheme to reduce the cost of configuring a radio communication system, a distributed antenna system (DAS) has been implemented. In the distributed antenna system, a signal processing device that processes a transmission signal and a radio device that outputs a radio signal are separated. In the following description, the signal processing device may be referred to as a “digital processing unit”. The radio device may be referred to as a “remote radio unit (RRU)” or a “remote radio head (RRH)”.
A transmission between a digital processing unit and a remote radio unit is implemented by, for example, Radio over Fiber (RoF). A radio frequency signal (RF signal) or an intermediate frequency signal (IF signal) is transmitted via an optical fiber in Radio over Fiber.
The digital processing unit generates an RF signal by, for example, up-converting a data signal. In this case, the digital processing unit converts the RF signal into an optical RF signal, and transmits the optical RF signal to the remote radio unit through an optical fiber. The remote radio unit converts the received optical RF signal into an electric signal and amplifies the electric signal. Then the remote radio unit transmits the amplified RF signal to a mobile station via an antenna.
In order to increase a transmission power of the RF signal to be transmitted to the mobile station via the antenna, the remote radio unit is equipped with a high-power amplifier that amplifies the RF signal. Here, the high-power amplifier may be controlled to operate near a saturation region so as to improve an amplification efficiency. Thus, a function to compensate for or suppress a distortion of an output signal of the amplifier is requested. As an example, a feedback signal that indicates the output signal of the amplifier is transmitted to the digital processing unit. Then the digital processing unit corrects a transmission signal so as to compensate for or suppress a distortion component of the output signal of the amplifier by using the feedback signal. As a result, the remote radio unit can output a signal in which a distortion is suppressed. Note that this function may be referred to as predistortion. A circuit that performs predistortion may be referred to as a predistorter.
In the above described system, the signal processing (including predistortion) is performed in the digital processing unit. Thus, the configuration of the remote radio unit can be simplified. As a result, the total cost for implementing a communication system may be greatly reduced especially in a configuration in which a plurality of remote radio units are connected to the digital processing unit.
Note that a transmission system in which a predistorter and a power amplifier are separately located is described in, for example, Japanese Laid-open Patent Publication No. 2014-103571. In addition, the related technologies are described in documents 1-3 below.    Document 1: H. S. Chung et al, “Design of RoF based Mobile Fronthaul Link with Multi-IF Carrier for LTE/LTE-A Signal Transmission,” IEEE MWP/APMW 2014, Sapporo, Japan.    Document 2: Changyo Han, Seung-Hyun Cho, Hwan Seok Chung, Sang Soo Lee and Jonghyun Lee, “Experimental Comparison of the Multi-IF Carrier Generation Methods in IF-over-Fiber System Using LTE Signals,” MWP 2014, Sapporo, Japan.    Document 3: Lei Ding et al., “A Robust Digital Baseband Predistorter Constructed Using Memory Polynomials,” IEEE Transaction on Communications, Vol. 52, No 1, January 2004.
In order to increase a capacity of a wireless link, a multi-antenna system that transmits a plurality of radio signals using a plurality of antennas is implemented. As an example of the multi-antenna system, a plurality of radio signals are transmitted from a plurality of transmitting antennas and received by a plurality of receiving antennas in a MIMO (Multi-Input Multi-Output) system.
In addition, a remote radio unit equipped with a plurality of transmitting antennas is proposed. In this distributed system, a plurality of signals are transmitted from the digital processing unit to the remote radio unit through an optical fiber. The remote radio unit respectively amplifies the signals and then outputs the amplified signals. However, a method to efficiently perform predistortion for the plurality of amplified signals in the distributed system is not known. Note that since a plurality of optical fibers are necessary to transmit a plurality of feedback signals that respectively indicate output signals of the plurality of amplifiers from the remote radio unit to the digital processing unit, a cost for configuring a communication system may increase.